Certain metallic molybdate and phosphate salts have been recognized for their ability to inhibit corrosion of ferrous substrates when incorporated as pigments in conventional and special coating formulations. Selected metal molybdate salts not only possess excellent corrosion inhibiting properties, but also provide advantages due to their non-toxicity and their neutral or white color, providing increased latitude in the formulation and use of such coating systems. In contrast, the more common corrosion inhibiting pigments, such as zinc chromate, basic lead silico chromate, dibasic lead phosphite and the like, are relatively restrictive in use due to their toxicity and/or their inherent dark colors. The effectiveness and desirable characteristics of typical molybdate-type corrosion inhibiting pigments are more fully described in U.S. Pat. Nos. 3,726,694 and 3,874,883, the teachings of which are incorporated herein by reference. In accordance with the teachings as set out in the aforementioned patents, the cost disadvantages normally associated with molybdate-type pigments relative to the more conventional prior art corrosion inhibiting pigments have to a great extent been overcome by various extender techniques enabling the use of substantially lesser quantities of the effective molybdate constituent to achieve equivalent corrosion inhibiting properties.
The present invention provides still further benefits in the use of such molybdate-type pigments by increasing their cost-effectiveness as a result of the discovery that superior synergistic corrosion inhibiting properties are derived by the combination of selected metallic molybdate salts with controlled proportions of selected metal phosphate salts without any sacrifice in the remaining beneficial properties of such molybdate pigments.